Various methods and apparatus have been proposed for actively reducing airborne noise transmitted through a structure. The general concept consists of introducing control vibrations into the vibrating structure to combine with the incident, noise transmitting vibration field existing in the structure such that they result in lower vibrations and significantly lower transmitted noise.
Sound transmission through a structure and particularly, sound transmitted through a panel subject to airborne noise is, generally, dominated by the mass-controlled, acoustically-fast, non-resonant modes of vibration of the structure. This is due to the spatial coupling of the lower-order structural modes of vibration with the excitation field (airborne noise) affecting the structure. The lower frequency, longer wavelength modes of vibration in the structure, when excited at frequencies much higher than the lower order resonant frequencies of the structure, become acoustically fast and radiate sound much better than the resonantly excited modes of vibration. As a result, the lower-order, non-resonant modes of vibration carry the majority of incident energy and noise through the structure and the resonant modes carry a lesser amount of the acoustic energy being transmitted.
The most important non-resonant mode for a planar structure or panel is the first non-resonant mode (1, 1). In general, a panel will vibrate at its first non-resonant mode when it is excited at a frequency higher than its own resonant frequency and when its modal wave number equals the acoustic wave number at the frequency of the excitation field. This first mode is always the first to become acoustically fast. If the frequency of the excitation field acting on the panel increases, higher order non-resonant vibration modes will also become acoustically fast as their modal wave numbers equal or exceed the acoustic wave number at the excitation frequency.
The prior art has pursued the active control of airborne noise transmitted by a vibrating panel. For example, the reduction of noise transmitted into an aircraft cabin by a vibrating interior panel. In this application, active noise control techniques have been developed for reducing the aircraft cabin noise levels at a particular tone. Most of these techniques function by determining the excitation frequency of the particular tone and then attempting to cancel that tone. Vibrations are then induced into the panel to generally oppose the particular excitation frequency. However, these techniques are only effective at reducing repetitive or periodic noise at a particular frequency or tone and do not address the cancellation or reduction of broadband noise transmitted through the panel.
More recently, prior art techniques have utilized piezoelectric materials to actively sense the incident vibration in a structure or panel as well as to counter those vibrations. However, these approaches have only been attempted for controlling tonal or periodic noise within a narrow frequency range and not for reducing noise having a broadband frequency range. Additionally, these approaches use a complex adaptive controller as the centerpiece of the global noise control strategy. These complex adapters are bulky, expensive, and inefficient.
Thus, there is a need for an apparatus and method for reducing airborne noise having a broadband frequency range transmitted by a vibrating structure. There is also a need for an apparatus and method for reducing noise transmitted through a vibrating structure where the frequency of the vibrations are broad and are rapidly changing. In addition, there is also a need for such an apparatus and method that is inexpensive, easy to incorporate and simple in operation.